A known process for introducing a t-butoxycarbonyl (hereinafter, also referred as “Boc”) group into a primary or secondary amino group consists of reacting the primary or secondary amine compound with phosgene or a phosgene equivalent in the presence of an organic base, followed by reacting with t-butanol. Since the phosgene or phosgene equivalent used in the reaction can be acquired inexpensively, the process is industrially useful.
Patent Document 1, for example, describes the obtaining of a Boc compound represented by the following formula (C) by adding triethylamine to the aniline compound represented by formula (A) in the presence of triphosgene and in a toluene solvent to convert to the isocyanate compound represented by formula (B), followed by adding t-butanol and triethylamine to the toluene solution of the isocyanate compound represented by formula (B).

Non-Patent Document 1 discloses the obtaining of a target Boc compound represented by the following formula (E) at a yield of 87% by adding triphosgene to a tetrahydrofuran solution of a compound represented by formula (D) at 0° C., followed by adding diisopropylethylamine and finally adding t-butanol.
